Explain lift-to-drag ratio and its importance for cruise efficiency.

• A. L/D measures aerodynamic efficiency; higher L/D yields better glide ratio, range, and fuel efficiency.
• B. L/D measures engine power per drag; higher L/D reduces range.
• C. L/D is the ratio of lift to density; higher L/D means heavier airplane.
• D. L/D is lift divided by wing area; higher L/D increases stall speed.

Answer: (A)

Lift-to-drag ratio shows how efficiently an aircraft uses lift relative to the drag it experiences. It’s the lift produced by the wings divided by the drag the airframe creates. In steady cruise, lift must balance the airplane’s weight, so the drag the aircraft must overcome largely governs how much thrust and fuel are needed to maintain that speed. When the L/D value is high, you get more lift for every bit of drag, meaning you can fly at the same weight and speed with less thrust and less fuel. That directly improves cruise efficiency and extends range.

A handy way to think about it is the glide ratio: in unpowered flight, the horizontal distance you can travel for each unit of altitude you lose is essentially the L/D. A higher L/D means you can glide farther, which mirrors how much distance you can cover per amount of fuel during powered cruise. At cruise, there’s an operating point where L/D is maximized for the given configuration, and flying near that point minimizes drag for the needed lift, leading to better fuel efficiency and longer range. In formal terms, range grows with L/D (all else equal), so improving L/D is a primary route to more efficient cruise.